T lymphocytes play a central role in the adaptive immune response to antigen. Naive T cells require two signals for their full activation (Bretscher 1999, Proc Natl Acad Sci USA 96:185-90). The first signal is antigen-specific and is provided by interaction of the T-cell receptor (TCR) with MHC/peptide complex on an antigen-presenting cell (APC). The second signal is a costimulatory signal provided by the interactions between receptors on the T cell and their ligands on the APC. Engagement of both TCR/MHC and co-stimulatory interactions leads to T-cell activation via a number of intracellular pathways, including calcium-calcineurin and RAS mitogen-activated protein kinase, and subsequent activation of transcription factors for a number of effector compounds, including cytokines such as IL-2. These events lead to T-cell proliferation, generation of a CD4+ helper T-cell (TH) pool, and expansion of activated CD8+ cytotoxic T cells. Not only is co-stimulation critical for full T-cell activation, its absence during TCR/MHC engagement results in anergy and/or apoptosis.
Although multiple positive and negative costimulatory pathways are involved in T-cell regulation, the most critical are between CD28 on T cells and B7-1 (CD80) and B7-2 (CD86) on APCs. CD28 promotes T-cell differentiation into TH1 phenotype cells and enhances antibody production by B cells and activation of T cells. B7-1 and B7-2, expressed on APCs such as dendritic cells (DC) and B cells, have overlapping but distinct functions. B7-2 is constitutively expressed and is rapidly upregulated on APCs coincident with TCR/MHC engagement (signal 1). B7-1 expression is very low on the resting cell, but is typically induced after prolonged T-cell stimulation. These differences suggest that while B7-2 may be important in initialization of T-cell activation, B7-1 may play a greater role in perpetuating the immune response.
Subsequent to T-cell activation, a negative regulatory receptor Cytotoxic T-Lymphocyte Antigen 4 (CTLA4 or CTLA-4, also called CD152), is upregulated on T cells (Alegre et al., 2001, Nat Rev Immunol 1:220-8). CTLA4 is structurally homologous to CD28 but binds more tightly to both B7-1 and B7-2 ligands. CLTA4 inhibits the immune response in two principal ways—it competes with CD28 for the B7 ligands and thus blocks costimulation, and it also negatively signals to inhibit T cell activation (Krummel and Allison, 1995, J Exp Med 182:459-465; Walunas et al., 1994, Immunity 1:405-413). Recent work has shown that B7-2 engages CD28 more than CTLA4 at the immune synapse, while B7-1 ligates more CTLA4 than CD28 (Collins et al., 2002, Immunity 17:201-210; Jansson et al., 2005, J Immunol 175:1575-1585).
Because of the critical role of the B7 co-stimulatory pathway in promoting and maintaining immune response, therapeutic agents designed to antagonize it are promising for the treatment of autoimmune diseases and disorders. Abatacept (Orencia®), is a CTLA4-Ig immunoadhesin consisting of the extracellular binding domain of CTLA4 linked to the Fc domain of a human IgG. Abatacept was developed to inhibit B7-mediated costimulation (Bluestone et al., 2006, Immunity 24:233-238), and is approved for the treatment of rheumatoid arthritis (RA) and in clinical trials for a number of other autoimmune indications. However, whereas abatacept shows some activity in treating RA, it is not effective for other indications. For example, CTLA4-Ig is far less efficacious in tolerance against transplant rejection (Kirk et al., 1997, Proc Natl Acad Sci USA 94:8789-8794; Levisetti et al., 1997, J Immunol 159:5187-5191).
The deficient clinical performance of abatacept has been attributed to its suboptimal affinity of native CTLA4 for the B7 ligands, particularly B7-2 due to its presumed importance in the initiation of immunity. The present invention provides novel variant CTLA4-Ig immunoadhesins with improved B7 affinities and enhanced T-cell inhibitory activities. Such novel immunoadhesins are of benefit in a variety of applications, particularly for treating immune related disorders, as discussed in detail below.